CAMBEIAX IX CAERNAEYOXSIIIRE. 259 



attention. We get them, in perhaps not their most characteristic 

 form, in several exposures round Caerhun, where they may be either 

 above or below the conglomerate, which is here lost sight of. South 

 of this a wide area is probably occupied by them, but nothing is 

 seen till we come to the quarries at Fachell which I have already 

 described, one of which has this well-marked type and another the 

 muddy conglomerate at the base. 



Now, at this spot there cannot be any doubt as to where we are 

 in the series, and we are thus prepared to examine Dinas Dinorwig 

 over the other side of the Silurian strip. All the north-western 

 slopes of the hill down to the fault are occupied by this Laminated Grit 

 (see fig. 5, p. 250), which, now we know it, is perfectly unmistakable. 

 It is on the south-eastern slopes and a little over the crest on the sides 

 that we find the conglomerate referred to by Dr. Hicks, ^ not only 

 in irregular crags which might be loose blocks, but also striking 

 solidly across the road on either side of the hill. This, then, is the 

 equivalent of the TairfFynnou Conglomerate, and does not lie at the 

 base of the series. In one part towards the east it partakes some- 

 what of the character of the Bangor Breccia, and in another there 

 is a band of felsite with conglomerate overlying it. Between here 

 and Pontrhythallt the exposures seen are mostly of the same blue 

 grit, though one quarry shows a conglomerate, as noted by Dr. 

 Hicks. As a whole, however, the general stratigraphy, and the 

 dying off of the Dinas conglomerate on its strike at iS'ant Efa, 

 indicate a fault, with the beds above the conglomerate undulating 

 as far as seen. 



The only other locality where these lower beds are seen is in the 

 Felin Hen cutting and to the south of it (see fig. 7, p. 253). The 

 Tairffynnon Conglomerate is here magnificently exposed ; in fact, 

 it is better shown than anywhere else. The pebbles are large and 

 in great quantity. It is curious, however, that there seems here to be 

 little or no interval between these beds and the Bangor Breccia, which 

 is the converse of what we last noticed near Pontrhythallt. The 

 dip of the beds is to the east, as in all the other rocks of the section, 

 and we accordingly find masses of the same rock at the farms of 

 Moel-y-Ci and Ty'n-y-Clau on the south, after which it appears to 

 die out. Below this conglomerate in the Felin Hen cutting comes 

 the Blue Laminated (irit, in quite a typical form, with the same dip as 

 the rest, and this again may be traced southwards if we go as far 

 as the rising ground at Careg-y-Fedwen, though the mapping here is 

 rather doubtful in detail. 



§ 10. The Beithdie Grit. 



This lies immediately on the felsite near Brithdir and round its 

 edge to the Caernarvon road ; it is therefore the lowest of all the 

 Cambrian series in this locality, and its character is already well 

 known. A similar small-grained quartz -felspar grit is seen at 



1 Quart. Journ. Geol. Soc. vol. xl. (1884) p. 197. 



